There are a couple different ways to copy the presentation details, renderings, layouts, etc. from one item to another item without duplicating A and manually resetting the content field values.
The presentation values of Item A are stored as an XML construct in the raw values of the Renderings field. You can see that here:
If you follow the blue arrows and click on Raw Values you'll see:
Raw Values Method
Select the rendering field value, copy and then in Item B, do the same thing, but PASTE the field value there.
Turn off Raw Value, and you should see the presentation on both Items now.
You can also use the Copy To dialogue as well. Which has been there forever, and I always forget about it.
There is an architectural viewpoint that must be considered. Generally, adjusting presentation on Items directly is frowned upon, because you break the association that it might have with the Template Standard Values that the Item derives from.
In those cases, generally you only muck with Final Renderings on the item itself, and Renderings on the Template Standard Values that the Item is made from.
Check out more information on how to edit the layout of an item, in Sitecore documentation here.